Electrical resistance element



1-946. P. ROBINSON ETAL 2,405,449

ELECTRICAL RESISTANCE ELEMENT I Filed Dec. 51, 1943 2 Sheets-Sheet 1 PRESTON ROBINS ON 0% STANLEYQ DORST I INVENTORS ATTORNEY Aug. 6, 1946. P. ROBINSON ETAL I ELECTRICAL RESISTANCE ELEMENT Filed Dec. 31, 1943 V 2 Sheets-Sheet 2 1&9. .5.

- vemwamsz INVENTORS m m I M w. 1 1

Patented Aug. 6, 1946 ELECTRICAL RESISTANCE ELEMENT Preston Robinson, 'illiamstown, and Stanley 0.

Dorst,

North Adams, Mass,

assignors to Sprague Electric Company, North Adams, Mass, a corporation of Massachusetts Application December 31, 1943, Serial No. 516,328

This invention relates to improved electrical resistance elements and more particularly refers to vitreous carbon resistance elements especially adapted for commercial use.

Electrical resistance elements are well known and comprise substances which offer a high resistance to the flow of electric current. There are many substances which are included in this category. Certain metals, and metal-alloys are used, for example, nickel-chromium, tungsten, etc.

Some non-metallic substances such as carbon make suitable resistance elements. Another type consists of ceramic bodies in which are suspended particles of carbon, metals or metal alloys. Electrical resistors produced with substances of the first class generally consist of wire,,for example, Nichrome wound around a porcelain or other ceramic rod or tube, with an air spacing between adjacent turns. This type of resistor is expensive to manufacture and requires a large volume per unit resistance. If wire wound variable resistors are made, difliculties arise from the fact that the wire turns are liable to move laterally along their core, thereby short-circuiting one or more turns and, further, the Wire itself will wear and alter the resistance.

Another type of resistor used is one manufactured with very fine insulated Nichrome wire. The insulation consists generally of fiber-glass, ceramic with or without resin impregnation, or plain wire enamel. Such resistance elements can be made with exact resistance values, but are fragile and generally have to be covered with a hermetically sealed glass and/or metal casing. They are expensive and, due to the above-mentioned casing, require a large volume per unit resistance. When they are not hermetically sealed, corrosive atmospheres alter the resistance unfavorably,

Potentiometer resistors, particularly for such specialized applications as volume controls for radio receivers, must meet certain important requirements. For example, Wear of the resistor element by the adjustable contact must be reduced to a minimum and preferably be nonexistent if the potentiometer element is to maintain its manufactured resistance value. Another important requirement is that a positive contact must be provided between the slider and the resistance element to avoid a high resistance contact which causes localized heating of the resistance element and unpredictable erratic changes in the adjusted resistance value in the 13 Claims. (Cl. 201-75) case of potentiometers handling large currents.

This is true also in the case of potentiometers handling small currents, i. e., volume controls for radio receivers and the like, as it produces similar 1 changes in adjusted resistance value accompanied by the generation of transient efiects such as noise in the accompanying circuits. In general, to meet these latter requirements it has been necessary to provide a high contact pressure between the slider and the resistance element. However, such an expedient reduces considerably the life of the potentiometer due to the additional friction and the corresponding wear of the resistance element So introduced.

In many instances potentiometers are required but inherently poor since their resistance value is subject to wide variation. Further the graphitic carbon is relatively soft and graphite resistors cannot withstand rough physical and/or chemical treatment. The resistance of these units can be increased easily by scraping or chipping away portions of the resistor. This expedient, however, exaggerates even more the undesirable effect of temperature upon the resistor. The resistors made of pure graphite, whether in the form of a rod or a surface deposit, requires a fairly large volume per unit resistance as the resistance of graphite itself is fairly low. These resistors cannot be used where accurate and dependable resistance is demanded.

The third type of resistor consists of carbon or metallic particles embedded in a matrix of ceramic or plaster material. By proper selection of materials and proportions thereof, high resistance elements may be manufactured. However, it is diflicult to obtain accurate resistance elements or to lower the resistance to a specified value, once the element is produced. The temperature coefiicient of resistance of these resistors suffers the same fault as the grapliitic carbon resistors, being erratic and high.

It is an object of this invention to produce resistance elements which overcome the foregoing disadvantages and others which directly or indirectlyresult therefrom. It is a further object to produce electrical resistance elements which possess new and desirable properties without being subject to the disadvantages of prior art resistance elements. It is a still further object to produce electrical resistors which will maintain substantially unchanged their initial resistance values over extended periods of time and under widely varying conditions. A still further object is to produce electrical resistance elements which can be manufactured to give predetermined resistance values. t is a still further object to produce electrical resistance elements which are incorporated in situ with other substances and materials to provide single unit electrical networks. An additional object is to produce noisefree, weather-proof insulators. Another object is to produce electrical resistance surfaces which are unaffected by conditions destructive to customary electrical resistance surfaces. A still further object is to produce electrically-conducting surfaces which are unafiected by wear, moisture, and other conditions which would injure prior electrically-conducting surfaces. A still further object is to produce electrical resistance elements with predictable resistance values and curves thereof per unit length. Another object is to provide light-absorbing surfaces which are neutral to different wave-lengths of light. Additional objects will become apparent from a consideration of the following description and claims.

These objects are attained in accordance with the present invention wherein vitreous carbon is used as the electrical resistance element. In a more restricted sense this invention is concerned with electrical resistance elements comprising a substantially non-conducting material upon which vitreous carbon may be deposited, a portion at least of the surface of which is coated with said vitreous carbon, as well as electrical circuits and devices containing layers of said material. In a still more restricted sense this invention pertains to electrical resistance elements comprising a refractory base coated in part with Vitreous carbon and partially short-circuited by imbedding bands of electrically conducting material in the vitreous carbon coating. In a still more restricted sense thi invention is directed to electrical resistors comprising a ceramic rod or similar material coated with vitreous carbon and partially short-circuited by means of bands of iron, aluminum or similar electrically conducting materials imbedded in the vitreous carbon. This invention is also concerned with processes for the production of the foregoing and related articles wherein precise resistance values may be obtained by suitable regulation of the amount and position of the vitreous carbon coating and the amount and position of the short-circuiting elements. This invention is also concerned with electrical insulators which are rendered noise-free and weather-proof by applying thereto vitreous carbon coatings,

In accordance with the present invention resistor and potentiometer requirements are met by a novel resistor or potentiometer construction in which the resistance element has a hardness approaching that of a diamond and a smooth mirror-like surface, whereby the hardness of the resistance element substantially entirely precludes wear thereof and its smooth mirror-like surface minimizes wear, in the case of the potentiometer, of the slider member. Furthermore, the resistance element is stable at relatively high temperatures and this factor coupled with a construction characterized by a high thermal conductivity makes possible resistors and potentiometers of a higher wattage rating per given volume than could heretofore be realized.

Vitreous carbon is not a new substance. Every student of quantitative chemical analysis is acquainted with the fact that when a hot glazed porcelain crucible, say of about pink heat, comes in contact with the blue portion of a Bunsen burner flame, a grayish-black, shiny film deposit is made on the crucible surface and that this deposit is exceedingly difficult to remove, if at all possible. This deposit has been called vitreous carbon, inasmuch as it possesses a high polish as in vitreous ceramic compositions, as well as extraordinary hardness.

Generally speaking, vitreous carbon may be formed when a relatively cool hydrocarbon comes in contact with a surface, the temperature thereof being from about 650 C. to about 2000 C. The hydrocarbon pyrolyzes and the carbon is deposited in a smooth crystalline layer rather than the usual soot which forms at lower tempera tures.

Examination by different means such as X- rays, micro-photography and the like have indicated that the actual crystalline structure of the carbon s0 deposited is like that of graphite, but that the crystal size is considerably less. The particle size range is from about 60 Angstrom units to about 300 Angstrom units, while graphite particle sizes are on the order of about 450 Angstrom units or greater. The intermediate range particles of sizes between 300 and 450 Angstrom units are known as retort-graphite. As indicated above the crystalline form of vitreous carbon is like that of graphite as contrasted to the crystalline form of diamond. However, the chemical and physical properties of vitreous carbon approach those of diamond much more closely than those of the graphite form of carbon.

Vitreous carbon, in particular, when deposited on surfaces of specific materials to be hereinafter enumerated, at temperatures in the range of about 650 C. to about 2000 0., possesses unusual hardness and chemical resistance. Vitreous carbon, formed at 1300 C., with particle sizes of about Angstrom units, is not scratched by corundumthus giving a hardness greater than 9 (by Mohs table of values, 10 being the hardness of diamond). Surfaces formed at 900 C. are about as hard as quartz, while surfaces formed at 1100 C., are about as hard as topaz, these being hardness values of about '7 and 8 respectively. It is apparent that there is a great difference between such surfaces and graphite surfaces, the latter possessing hardness values from about 0.5 to 1.5at the other extremity of the hardness scale.

The chemical stability of vitreous carbon is likewise similar to that of diamond. A surface leyer of vitreous carbon will withstand the attack of reagents which will readily attack graphite. For example, there is no reaction of vitreous carbon with boiling concentrated sulfuric acid, hot mixed acids, such as nitric-sulfuric, fused sodium sulfate and other reagents which will destroy graphite. In fact this stability makes it possible to clean by drastic means the surfaces of electrical resistance or other elements made therefrom, re.- moving all foreign substances thereon, such as soot or other impurities, without in the least a1- tering the vitreous carbon.

In relation to electrical properties, the vitreous carbon is similar to diamond. The electrical resistivity per unit volume is from eight to twenty times that of graphite, depending upon the temperature of vitreous carbon formation, and upon thequality or type of graphite with which it is compared.

The physical appearance is similar to that of a polished metal surface, particularly when the vitreous carbon is deposited on a glazed surface The elasticity of the vitreous carbon is unusual, in that it is sufficient to allow the vitreous carbon layer to expand directly with the lineal or other expansion of the surface upon which it is deposited. In accordance with this invention, use is made of this novel property. The coeflicient of linear expansion per unit length per degree C. is less than the approximate value 7 10 of graphite, but greater, however, than the value of 1 X 10 of diamond.

In the manufacture of vitreous carbon electrical resistance elements, it is advisable that the portion of the assembly comprising the electrical resistance contain no metallic particles or surfaces, since most metals offer little resistance to the flow of electrical current. While vitreous carbon itself, as heretofore explained, possesses high electrical resistance, it is obvious that this property may be nullified if the vitreous carbon is deposited on a metallic electrical conducting surface or is contaminated with an abundance of metal electrical conducting particles. Therefore, in producing electrical elements, it ha been found that certain ceramic materials make highly desirable deposition surfaces.

In general, vitreous carbon layers are formed by reacting a hydrocarbon gas with a hot surface under such conditions that the resulting vitreous carbon does not react further with the surface upon which it is deposited to form carbides thereof or other detrimental reaction products. This feature is essential, as otherwise the resulting carbides Will favor the formation of undesirable soot rather than a vitreous carbon layer. By depositing vitreous carbon under the foregoing equilibrium conditions materials which would otherwise form carbides and be worthles for this purpose may be used.

In the case of a reducible oxide, such as magnesium oxide, at low rates of deposition of the vitreou carbon, at secondary reaction takes place to produce metallic Mg which distills away. At high rates of deposition, which are high compared to the rate of reduction, it is difiicult to find any trace of the secondary reaction. In the same manner, aluminum oxide may be reduced and forms aluminum carbide if vitreous carbon is deposited thereon at temperature above 1,o0o C. However, if vitreous carbon is deposited on aluminum oxide at lower temperatures, no reduction or carbide formation occurs, so the resulting coated product is of particular value as a resistor element.

The surprising phenomenon referred to previously permits the formation of vitreou carbon layers on surfaces which were heretofore considered to be of no use for this purpose. Surfaces so coated with layers of vitreous carbon are then of outstanding value as resistor elements.

The surprising fact that excellent resistance elements may be formed by depositing vitreous carbon on ceramic surfaces of alumina, mullite, aluminum silicate and aluminum oxide-silica compositions despite the possible formation of aluminum carbides, herein referred to a carbideforming materials, is taken advantage of in the present invention. This phenomenon i contrary to the generally accepted understanding thatvitreous carbon layers could not be formed' on surfaces which react to form carbides.

Another factor of importance in selecting base linear expansion of the vitreous carbon itself is.

small, and it therefore expands, with thermal increases, with the base upon which it is deposited.

If the expansion of the base is unduly large, the.

layer of vitreou carbon may stretch and cause variation in the resistance value. Some base materlals qualify very well in both chemical construction and thermal expansion coefiicients;.

among these are quartz, fused quartz glass, alundum, and mullite.

Among the various surfaces upon which vitreous carbon layers may be formed with excellent results are the following:

Carbon, graphite, Pyrex glass and boro-silicate glasses, porcelain, sintered aluminum oxide,

aluminum carbide, silicon carbide, aluminite,-

stronmullite, steatite, Isolantite, barium-, tiumand other titanates, bentonite, kaolinite, minerals such'as andalusite, keulandite, etc; muscovite and other mica minerals, quartz, and various other similar materials whichare stable at the temperatures required. The surface may be glazed or unglazed, as desired. The characteristics of the resulting coating remain the same. except in physical appearance.

The vitreou carbon may be deposited on the heated base by passing a hydrocarbon gas over the surface thereof. The nature of the reaction makes advisable an atmosphere which is not strongly oxidizing. The temperature of the base should be sufficient to pyrolize the hydrocarbon and the resulting carbon aifixe itself to the sur face in vitreous form. Generally, an inert gas is admixed with the hydrocarbon and the flow of both hydrocarbon and inert gas is usually continuous.

The inert gas, if used, serves several purposes. First, it provides a means of vaporizing the hydrocarbon, as hereinafter explained. Second, the pressure and volume of the inert gas may control the rate of reaction and vitreous carbon formation. Third, the inert gas serves to sweep 01ft the reaction chamber before and following the reaction as Well as during the actual operation. Fourth, the dilution of the reactant hydrocarbon may be preferred so that too thick, uneven and spotty layer may be avoided, as well as too. high a reaction rate. In addition to the inert gas, small amounts of other gases may be used. Among these is oxygen. Use of the oxygen in amounts generally less than 20% of the hydrocarbon may oxidize the soot or other amorphous carbon which may form along with the vitreous carbon.

Another constituent which may advantageously be added is water vapor.

Obviously excessive oxygen would decompose the vitreous carbon and an excess thereof should preferably be avoided. Carbon dioxide and nitrogen are both chemically inert under the conditions of deposition and may be used satisfactorily as inert gases, since they are readily available and cheap. Other inert gases may be used, however, with satisfactory results. The hydrocarbon used may be a single compound or a mixture of several compounds. Generally speaking any organic compound providing carbon upon pyrolysis may be used, as long as such compound does not possess a sufficient amount of oxygen to oxidize all the carbon formed. Preferably, however, hydrocarbon are used, particularly saturated hydrocarbons. Among the hydrocarbons which can be used are the following representative compounds: methane, ethane, propane, butane, pentane, hexane, heptane, octane, nonane, decane, dodecane and longer chain compounds of the same series; ethylene, butylene and derivatives thereof; natural gas, acetylene and derivatives thereof; gasoline and petroleum products; ligroin, benzene, toluene and related benzene derivative-s; naphthalene, anthracene, and alcohols, aldehydes, ketones and various other organic compounds. Isomers of such compounds may be used. It is, of course, understood that mixtures of the foregoing and/or related compounds may be used.

The above compounds may exist in any of three states at room temperature, 1. e., gaseous, liquid and solid. Provision must be made to obtain the gaseous phase of the hydrocarbon or other organic compound prior to contact with the heated surface. If the compound is a solid, it may be heated to or above its melting point, then vaporized by heating to the boiling point or by passing or bubbling a stream of inert gas through the liquid. The latter method is generally preferred, since by controlling the temperature of the liquid and therewith the vapor pressure of the liquid componentand the pressure and flow of the inert gas, a controlled gas concentration and flow may be obtained. It may be desirable to preheat this gas mixture (or hydrocarbon) by exchanging heat from the exit gases from the reaction chamber. The temperature of the gas immediately prior to pyrolysis on the deposition surface is generally controlled so that it is too cool to pyrolize in and on the tube or pipe in which it flows and still warm enough to limit the cooling of the surface to be coated to a negligible extent. The space velocity is generally rather low, since more homogeneous, smooth and outstanding layers are formed with lower space velocities.

The reaction is generally carried out in a closed furnace, heat being transmitted through the walls from electrical heating units, gas fired chambers and the like. A preferred method is to mix the exit gases with air and allow this mixture to burn and heat the furnace. In this manner, a maximum heat efficiency is obtained. Another expedient which can be used is to place a heating element within the article to be coated, as, for example, inside a porcelain tube used for a resistor core. Any other suitable method of obtaining the desired temperature, which will ordinarily be in the range of about 650 C. to about 2000 C. may be used.

In customary practice, the furnace may be swept out with an inert gas to remove oxygen and air before the operation. The ceramic or other body or bodies to be coated are placed in the furnace and the temperature raised to 1300 C. The flow of hydrocarbon mixture is then begun, in a slow stream. The vitreous carbon is thereupon deposited upon the surfaces provided. When the desired layer thickness is reached, the gas flow is stopped and the furnace swept out with an inert gas. The cooling down of the coated materials to a temperature of about 600 C. or less, may be conducted in the inert atmosphere. This process may be batch or continuous in nature, de-

pending upon the furnace and equipment designed therefor. The deposit thus obtained may be from one molecule up to several millimeters in thickness. In the manufacture of electrical resistance elements, the thickness of the deposit governs the electrical resistance. The thinner layers possess higher resistance values than similar thicker layers.

As heretofore mentioned, the hardness of the vitreous carbon layer is extremely high. This is a desirable property, but at the same time, presents a problem when the vitreous carbon layers are used in electrical resistance elements, due to the fact that only a diamond will scratch away the vitreous carbon, if it is desired to change the resistance value. A preferred embodiment of this invention comprises methods of altering resistance values of the resistance elements made of vitreous carbon.

One method which may be used to obtain resistors of a definite resistance value is to incorporate an electrically-conducting metal in the vitreous carbon assembly, which partially shortcircuits the resistance unit, and subsequently adjust the resistance value of the assembly by removing part of the electrically-conducting material.

The short-circuiting element may be deposited before or after deposition of the vitreous carbon, preferably the former. This material should be electrically conducting and should be deposited or otherwise positioned on the base in such manner that it partially short-circuits the same and may be readily removed therefrom in order to increase the resistance value of the final product to the desired degree. Short-circuiting elements which may be used for this purpose are extremely varied; for instance, iron, aluminum, copper, and silver and the like. This short-circuiting coating may be applied by sublimation under vacuum, chemical deposition, or in other suitable manner for applying a conducting material to a nonconducting base in such form that the material may thereafter be readily removed by scraping, stripping, or chemical action thereon.

The foregoing electrically conducting material may be applied to the base in the form of helical or longitudinal strips or in any other desired form which will result in short-circuiting of the resistor to a sufficient extent to reduce it below the ultimate desired value. Here again the amount of electrically-conducting material and the position thereof will depend to a great extent upon. the type of resistor and the use for which it is employed. No difficulty should be occasioned in selecting the proper material and depositing or otherwise positioning the necessary amounts thereof in view of theinstructions heretofore and hereinafter set out.

Where the electrically-conducting material is deposited before the vitreous carbon it is frequently helpful to select a material upon which vitreous carbon will not deposit, such as, for example, iron upon which a carbide may form. This may be subsequently removed by chemical action. In the alternative, a material may be used upon which vitreous carbon will deposit such as gold, bismuth, tin, silver, copper, platinum, but the material may be masked by covering it with asbestos, talc, chromium oxide or other masking layers which will prevent vitreous carbon from subsequently being deposited thereon. It is also contemplated that the conditions under which vitreous carbons are deposited may be so 9. selected that it will not be deposited upon the short-circuiting material.

The resulting resistor has its surface substantially coated with integral strips of carbon between which are one or more integral strips of an electrically-conducting metal or metals. Terminals are applied thereto in the customary manner and the product is tested to determine its resistance. This resistance should be lower than that desired in the final product. Thereafter the resistance is increased to the desired level by removing portions of the electrically conducting material from the surface of the resistor. Removal of this material may be accomplished in various ways, for example, by treatment with a suitable chemical agent, such as nitric acid, which has no effect on the vitreous carbon. In the alternative, the electrically conducting material may be removed by stripping or scraping portions of it from the surface. Removal of this material as aforesaid or by related methods is continued until the precise degree of resistance is obtained in the resistor. Thereafter a housing or protective coating may be applied to the outside of the resistor and it may be used for its intended purpose without further adjustment.

An alternative method is th deposition of a liquid metal such as liquid silver, upon the vitreous carbon layer. Such liquid metals consist of minute particles of silver and/or other metals, 3

which can be removed by scraping or by treat- 13 ment with an acid or other chemical reagent. The temperatures to which such liquid must be raised to form a satisfactory contact will have no deleterious effect upon the vitreous carbon surface upon which the metal is deposited. In 1 this manner, a layer of electrically-conducting metal can be painted in any desired position, raised to an elevated temperature, and subsequently removed by scraping or chemical means, during the final adjustment of the resistance element.

Another alter-native method for varying the resistance of a vitreous carbon unit is to place the coated unit in an atmosphere of oxygen or other destructive gas. By subjecting the unit in this atmosphere to a high frequency field, or to the passage of an electrical current, the resistance may be increased. With suitable apparatus the resistor may be placed in a testing circuit located in a high frequency field and with an oxygen atmosphere, with the circuit so arranged that when the resistance value increases to the de sired point, the high frequency field will automatically disconnect and no more vitreous carbon decomposition will transpire. There are many possible variations of this type of adjustment which may be used.

It has been heretofore mentioned that certain masking agents may be applied to portions of the article upon which the vitreous carbon is to be deposited, for example, asbestos, talc, and the like. Actually, the masking agents may receive vitreous carbon deposits, but by virtue of their chemical properties, physical weakness, and/or porosity, may be readily removed subsequently, removing therewith the vitreous carbon thereon. For example, asbestos may receive some deposit on the fibers, but there will be no chemical reaction between the asbestos and the ceramic or other surfaces, and the asbestos may be stripped off following the vitreous carbon deposition. If an irreducible metal oxide, such as chromium oxide is used, there will be no vitreous carbon deposition and the oxide may be removed by stron acids. Talc may be applied as a water paste, which, upon heating, loses water and becomes a porous mass upon the exposed particles of which the vitreous carbon will deposit, but which is sufficiently dense to prevent the vitreous carbon from depositing onthe surface beneath the talc. The physical weakness of the talc will allow removal by scraping, following th vitreous carbon deposition. In general, the following masking agents are suitable, fibrous and/ or porous inorganic materials such as asbestos or talc; carbide forming metals or metal oxides; heat-stable metal oxides, such as chromium oxide; and the like.

In some cases, as heretofore and hereinafter described, it is desirable to have a unit possessing a variation of vitreous carbon layer thickness and/or electrical resistance in the unit itself. An embodiment of this invention concerns the method of providing a controlled variable thickness vitreous carbon deposit. In accordance with the invention, a snug fitting metal or high temperature ceramic shield is provided for the body to be coated. By varying the position of the shield, certain predetermined parts of the ceramic body will receive more or less vitreous carbon than other portions. This variance may be by steps or by gradual changes in thickness. Generally, the shield will be constructed of the same material as the body which is to be shielded, since the thermal expansion will be the same. For example, if a mullite rod 2" long and A1, in diameter is to be coated with two different thicknesses of vitreous carbon, to give an element with 50,000 ohms resistance and 10,000 ohms resistance in series, a shield comprising a mullite tube, with inside diameter A" (plus a positive tolerance) would be provided. The shield would he slid over the high resistance end up to the middle of the mullite rod and the deposition continued for about the same length of time as that before the shielding, say one minute in a 10% methane-% nitrogen gas mixture at 1300 C. Following the deposition, terminals could be affixed to the ends and the middle of the resistor unit to provide the unit desired. The resistance values could be altered by use of the liquid silver or other suitable means to obtain precise values of resistance.

Gra hite shields can be used, since a tight fit is lubricated in situ, by the graphite. The unit can be dipped in acid or other chemicals following the process in order to remove from the surface all graohite particles that may adhere thereto. Metal shields may also be used, if machined to fit correctly at the desired temperatures.

The shape of the shield may be varied widely, depending upon the surface to be coated, for instance, it may be tubular, flat, helical, conical, half-spherical, etc., and may move laterally. about an axis, helically, etc., as specifically required.

Reference to the a pended drawings will further clarify the invention. In these drawings:

Figure 1 shows an insulating core partly in cross section provided with a helically-formed resistance adjusting coating in accordance with the invention.

Fig. 2 shows a variation partly in cross-section of the element shown in Fig. 1.

Fig. 3 shows an electrical resistor made in acoordance with the invention.

llll

Figure 4 is a cross-sectional view of a potentiometer constructed in accordance with the invention, and

Figure 5 is a side elevation of the potentiometer of Figure 4 taken along the line 55.

In Figure 1 a refractory ceramic core is preferably of mullite is provided with a coating H in the form of a helical spiral extending from one end of the core to the other and leaving the major portion of the core surface exposed. The coating ll consists of an electrically conducting material such as aluminum, copper, etc. The coating it may be applied in any Well-known manner, for example, by chemical deposition or by sublimation under vacuum. For example, when forming a coating of aluminum, this metal may be sublimed on the core i 9 by sublimation of aluminum heated to a temperature of the order of 1300" C. under vacuum of the order of 10 to 50 microns pressure. To restrict the metal deposition to a helical coating the surface voids of the core (shown as l2) are masked, for example, by means of a masking coating (not shown).

Over each end of the core it and imbedding the ends of the coating H are metal contact caps i3|3 of low resistance contact metal such as copper or silver. The caps lt-is may also be applied by chemical deposition or by sublimation; for example, silver may be sublimed over the ends of the core under vacuum at a pressure of the order of 10 to 50 microns. It is to be understood that to effect the deposition of the caps iB-ES it is necessary to remove the masking coating from the ends of the core. Similarly, to prevent deposition of the cap metal over the central portion of the coating 1 i, it is necessary to mask the coat ing prior to depositing the end caps.

Upon deposition of the coating l i and the caps l3l3 and the removal of the masking coating a structure such as shown in Fig. 1 is obtained.

The masking coating material used will be determined to some extent by the method of deposition used, and the expedient used to remove the masking coating is determined by the material of which it is composed. For example, masking i coating such as asbestos, talc powder, and/or porous or fluffy inorganic compounds may be used. These coatings are applied to those portions of the element which it is desired to protect from deposition in the ensuing deposition treatment. Thereafter, they may be removed therefrom by stripping or other suitable methods such as scraping, chemical action, and the like.

Fig. 2 is similar to Fig. 1 except that the electrically conducting material is in the form of a plurality of strips Ma extending parallel to the axis of the core across the length of the core.

The structure of Fig. l. or of Fig. 2 is now ready for deposition of the vitreous carbon material forming the resistance element. This may be accomplished as follows: The structure may be heated to a temperature of the order of 790 C. to 1300 C. in an atmosphere of a suitable hydrocarbon such as methane, propane. propylene, butane, isopropane, natural gas, petroleum, and the like, the latter being relatively cool. Upon contact of the cool hydrocarbon with the heated core, the hydrocarbon decomposes and deposits a vitreous carbon coating on the bare portions of the core. The thickness of the coating is determined by the desired resistance value of the resistor to be produced.

For adjusting the resistance value of the resulting vitreous carbon resistance unit to close tolerance, the element is placed in a suitable resistance measuring device and the coating H is progressively removed from the core, for example, mechanically, by scraping; and chemically, by dissolving in an acid such as nitric acid, which has no effect on the vitreous carbon. This is shown in Fig. 3 wherein the coating l l is removed from the surface of the core H) and from between the turns of the vitreous carbon coating I5 up to the point It.

From the above it will appear that by reason of the short-circuiting action of the coating l i, that portion of the resistance coating It to the left of the point it is effectively short-circuited and has a low overall resistance value, whereas that portion of the coating i5 to the right of the point it has a high resistance value, as determined by the length and cross sectional area of the said latter portion of the resistance element.

The adjusted resistance element is now ready for assembly into the resistor shown in Fig. 3. For this purpose the ends of the resistor are coat ed with conducting metal caps Il'-l'l formed in the manner of the caps [3-13. Terminal wires i8l8 are soldered or similarly secured to the end faces of the caps I'.'-ll and the assembly positioned in an insulating casing is of glass, porcelain, Isolantite or the like, resins, paper, etc., and secured therein by means of a suitable refractory cementing layer 2!! which upon hardening forms a closure for the casing it.

The potentiometer shown comprises a cylindrical metal container 3i? having a wall portion 3| and an end face portion 32, the latter being centrally provided with a tubular extension 33 threaded at 34. The inner surface of the portion 32 is provided with an annular coating 35 consisting of a refractory electrical insulating material such as glass, porcelain or the like. Preferably, but not necessarily, the coating consists of a potassium lead-silicate vitreous enamel as described in U. S. Patent 2,298,947 issued July 26, 1942, and may be applied by cataphoretic deposition and subsequent fusion as described in that patent. To prevent contamination of the insulating coating the container 30 preferably consists of or has a surface coating of a metal whose oxide is diiiicultly soluble in the enamel, as mentioned in the aforesaid patent. Suitable means for this purpose are iron, nickel, chromium, and alloys thereof, and particularly commercial pure soft iron.

Superimposed upon and integrally bonded to the coating 35 is a crescent-shaped annular resistance element 36 consisting of vitreous carbon formed by heating the refractory coating 35 to a temperature of the order of 700 C., and pyrolyzing thereon a relatively cool hydrocarbon gas such as methane, ethane, propane or the like. The deposition of the vitreous carbon element 36 is restricted to the shape described by suitably masking the remaining exposed portions of the insulating layer 35, for example, by a masking foil of a metal such as Nichrome which does not adhere to the vitreous insulating coating 35 at the temperature necessary for the deposition of the resistance element, or by a masking coating of a material which is preferentially removable with respect to the material of the insulating layer 35, which is subsequently removed together with any superposed deposits of vitreous carbon by dissolving in an acid in which the coating 35 is insoluble. The area covered by the vitreous carbon and its thickness will be detercontact or the like, but preferably by means of metallic deposits of silver, copper or the like such as shown at 31 and 38, which deposits may be formed by spraying the metal in finely-divided form, chemical precipitation from a suitable solution, sublimation under vacuum from a molten pool of the coating material or by cathodic sputtering. In practice it is preferable to coat the insulating coating 35 at the connection portions of the resistance element prior to the deposition of the-resistance element so that the subsequent application of theconnecting deposits 31 and 33 entirely embeds the end portions of the resistance element.

Externalelectrical connection to the ends of the resistance element are provided by means of terminals 39 and 49 each of which comprises a terminal lug 41 positioned on the outside of the container 30 and secured to a. tab 42 by a rivet 43 with the interposition of insulating washers '44-44 by which the terminal is insulated from the container 30. The tabs 42 may be conductively secured to the deposits 3|38 in any well known manner, for example, by soldering, brazing, welding or the like. 7

As the adjustable element of the potentiometer there is provided a slider 50 consisting of two concentric interconnected annular strips and 52 of ph'osphor-bronzeor similar springy electrical conducting material. The strip 5! is formed with an integral extending'shoe portion 53 adapted to pressure-contact'the surface of the resistance element 36. Preferably the contacting surface of the shoe portion 53 is provided with a coating of smooth metal or alloy such as osmium, brass, bronze, or the like, which in conjunction with the mirror-like surface of the resistance element reduces wear of the shoe portion to a minimum. V

The slider 50 is rotated over the surface of the resistance element by means of a shaft 55 jour naled within the portion 33 and to the inner end of which is secured a disk 56 of insulatin material such as Bakelite, hard rubber, Isolantite or the like, said disc being secured to the shaft by an integral rivet portion 51 of the shaft 55,

and to the slider 59 by means of rivets 6il-6fl engaging the strip 52. Axial movement of the shaft 55 within the portion 33 is prevented by means of a split washer 58 which abuts the end face of portion 33 and engages an annular groove 59 of the shaft.

Electrical connection to the slider 50 is provided by a phosphor-bronze washer Bl interposed between the disc 55 and strip 52 against one face of which bears a contact shoe 62, said shoe being electrically secured to an external terminal lug 63 by a rivet 64 passing through the wall 3| of container 30 and insulated therefrom by insulating washers 65-65.

A cover encloses the container 39 and protects the potentiometer component from dust and the like.

It is to be understood that the above instructions may be adapted to other types of variable resistors than that illustrated in the drawings. For example, rectangular, circular o cylindrical elements may be employed, in accordance with standard practice, and the mechanism for moving the contact over the vitreous carbon surface may be designed accordingly. Likewise, electrical tending into the resistor material.

contact may be made t either one or both ends of the vitreous carbon element. This may be accomplished by riveting a silvered brass strip against the band, by applying silver to the glaze before the Vitreous carbon is applied thereto, etc., or other methods heretofore mentioned. Numerous other modifications, embraced herein, will occur to those familiar with this art from a consideration of the above instructions.

It is also to be understood that the fixed resistors referred to previously may be varied widely in their design and construction without departing from the scope hereof. For example, instead of a strip of short-circuiting' metal being imbedded in the vitreous carbon other modifications thereof are contemplated, such as a metallic spike connected to one or both terminals and ex- By suitable treatment of this spike the resistance Value of the unit can be adjusted to any desired level.

' The finished resistors may be enclosed in a housing of ceramic, glass, paint, resinousmaterial, etc., if desired. Since moisture Will not decompose or alter the vitreous carbon, less precaution is needed in this respect than in the case of prior art resistors. Excellent results have been obtained by dipping the resistor in a solution of polymerized vinyl carbazole or other resin or resin-forming material r mixture thereof. In this connection, the instructions of copending application, Serial No. 475,051, filed by Lester A. Brooks, on February 6, 1943, are applicable and are of particular value.

Another embodiment of this invention is to coat graphite or other prior art resistor materials with a layer of vitreous carbon. In this manner the soft graphite is protected from mechanical abrasion, chemical action, etc., Without decreasing its resistance value.

An additional embodiment is the use of vitreous carbon in resistance units of the so-called tapped variety. By shielding portions of the ceramic surface during specified intervals in the deposition of the vitreous carbon, difierent thicknesses of the vitreous carbon may be obtained, to form diiferent resistance values in different section of the element. Terminals may be afiixed to the resistance element so that different resistance values may be obtained from the same resistance element. It has also been found possible to vary the thickness of the vitreous carbon layer at any desired rate along the vitreous carbon resistance element. This may be done, as stated previously, by adjusting the movement of a shield to conform to the thickness of vitreous carbon deposit in any ne portion of the surface. In this Way an excellent variable resistance unit, similar to the potentiometer described in connection with Figs. 4 and 5 may be produced to give a non-linear resistance characteristics, such as might be desired in a radio volume control to conform to the non-linear characteristics of the human ear.

The foregoing instructions are also applicable in the manufacture of electrical networks and the like, wherein one or more resistance elements is used in conjunction with capacitance and/or inductive elements. The physical and chemical properties of the vitreous carbon layers formed in accordance with this invention permits the use of vitreous carbon in a variety of networks, such as ph ase-inverters and the like.

It has also been found that by depositing a layer of vitreous carbon on the external surfaces of a terminal insulator, the insulator becomes noise-free as well as weatherproof. This former '15 property'may be attributed to the fact that the insulator is effectively shunted by an electrical resistor, and a noise-free as well as Weatherproof insulator and a suitable bleeder resistance are thereby produced in the same unit.

In other fields, the vitreous carbon layers formed in accordance with this invention are useful by virtue of their unusual chemical and physical properties. For example, deposits of vitreous carbon on glass may be employed as standard light filters because of the blackness and neutrality of Vitreous carbon to visible wave lengths of light. They may also be used as coatings on softer materials to prevent wear and/or friction. An example of this would be the use of a smooth vitreous carbon layer on the familiar graphite brushes used in electrical motors and generators. The hard smooth surface would be electrically conducting While at the same time it would not wear away as would graphite.

As many widely different embodiments of this invention may be made without departing from the spirit and scope thereof, it is to be understood that the invention is not limited to the specific embodiments thereof except as defined in the appended claims.

We claim:

1. An electrical resistance element comprising a non-conducting base coated with vitreous carbon, and partially short-circuited by electrically conducting material imbedded in said vitreous carbon.

2. An electrical resistance element comprising a refractory rod of aluminum-containing material coated with vitreous carbon, and partially short-circuited by bands of electrically conducting material imbedded in said vitreous carbon.

3. A process for producing a resistor element which comprises pyrolyzing a hydrocarbon gas upon the hot surface of a carbide-forming material under such conditions that carbide formation between the resulting vitreous carbon and the surface material is substantially prevented.

4. An electrical resistance element comprising a non-conducting base coated with vitreous carbon, and partially short-circuited by incorporating electrically conducting material with a portion of said vitreous carbon coating.

5. An electrical resistance element comprising a non-conducting base coated with vitreous car- LII.

bon, and partially short-circuited by incorporating a strip of electrically conducting material with said vitreous carbon coating.

6. An electrical resistance element comprising a non-conducting base coated with vitreous carbon, and Partially short-circuited by incorporating a strip of silver with said vitreous carbon coating.

7. An electrical resistance element comprising a non-conducting base coated with vitreous carbon, and partially short-circuited by incorporating a strip of copper with said vitreous carbon coating' 8. An electrical resistance element comprising a non-conducting cylindrical ceramic base coated with vitreous carbon, and partially short-circuitcd by incorporating a strip of electrically conducting material with said vitreous carbon coating.

9. An electrical resistance element comprising a non-conducting cylindrical ceramic base coated with vitreous carbon, and partially short-cirouited by incorporating a strip of silver with said vitreous carbon coating.

10. An electrical resistance element comprising a non-conducting cylindrical ceramic base coated with vitreous carbon, and partially shortcircuited by incorporating a strip of copper with said vitreous carbon coating.

11. A process for producing a resistor element which comprises pyrolyzing a hydrocarbon gas upon a hot ceramic surfac under such conditions that carbide formation between the resulting vitreous carbon and the surface material is substantially prevented.

12. A process for producing a resistor element which comprises pyrolyzing a hydrocarbon gas upon a hot cylindrical ceramic surface under such conditions that carbide formation between the resulting vitreous carbon and the surface material is substantially prevented.

13. A process for producing a resistor element which comprises pyrolyzing a hydrocarbon gas upon a hot cylindrical ceramic surface, said surface containing aluminum oxide and being heated to a temperature in the range between about 650 C. and about 1,000" C.

PRESTON ROBINSON. STANLEY O. DORST. 

